1. Field of the Invention
The invention relates generally to digital communication systems and, more particularly, to systems utilizing reference symbols for timing synchronization and frequency offset estimation.
2. Description of the Related Art
Digital communication systems typically utilize bandwidth-efficient modulation schemes to maintain high bit rates for a number of user channels. Such systems transmit bursts of information symbols that may include both reference symbols and data symbols. The reference symbols are known by a receiver to provide timing and frequency information, thereby assisting in the demodulation of the data symbols. Even though typical satellite digital communication systems may provide mechanisms (e.g., a random access channel (RACH) burst) for providing a receiver with ranges for the timing and frequency of a signal burst, such ranges are insufficient for demodulation. The actual timing and frequency must be identified within the given range. Moreover, the timing and frequency can change for a variety of reasons, including Doppler frequency shift due to satellite motion, propagation delays, and oscillator drift.
In order to demodulate the data symbols accurately, the clock in the receiver must be synchronized with the clock in the transmitter and, further, the oscillator in the receiver must be to aligned with the actual carrier frequency (i.e., the received carrier frequency). With certain modulation schemes (e.g., QPSK), the receiver is capable of deriving timing and frequency information from the information symbols themselves subsequent to removing the modulation. These conveniences are not available with modulation schemes such as GMSK (Gaussian Minimum Shift Keying), in which modulated information is transmitted over a plurality of bit periods, inasmuch as no single non-linearity exists for modulation removal. However, such memory-inducing modulation schemes provide several advantages, not the least of which is the constant envelope of the transmitted signal. This advantage is particularly useful for satellite communication systems, inasmuch as cheaper Class C amplifiers may be used in both the satellites and the receivers.
In the interest of accurately estimating the timing of the burst, past transmission formats have bundled the reference symbols in a unique word having data with considerable variation (such that the transmitted signal has high frequency components). On the other hand, to attain accurate frequency estimates, past transmission formats have set all of the reference symbols in the unique word to xe2x80x9c1xe2x80x9d to obtain high resolution for the frequency variable. Because both the timing and frequency offset of the burst must be determined, past signal bursts have included a single, lengthy preamble segment to provide a suitable number of reference bits for each operation. The length of the preamble, of course, comes at the expense of a lower data throughput rate.
In light of the above, it would be desirable to determine accurately both the timing and frequency of a burst from a minimum number of reference symbols transmitted therein, particularly in the context of constant envelope, bandwidth-efficient modulation schemes with reference symbols spanning a plurality of bit periods. It would also be desirable to make such a joint determination without having to estimate the carrier phase, signal amplitude, or noise level, and further without having to compensate for fading in the channel.
The method and apparatus of the present invention is useful in a digital communication system in which a signal burst is transmitted. According to one aspect of the present invention, a burst analyzer comprises a filter that generates correlation data based on the signal burst and a plurality of reference signals offset by a plurality of time offsets and an accumulator that combines quantities based on the correlation data generated by the filter for a plurality of reference segments distributed within the signal burst.
According to another aspect of the present invention, a method of synchronization in a digital communication system transmitting a signal burst includes the step of calculating correlation data based on the signal burst and a plurality of reference signals offset by a plurality of time offsets. The method also includes the step of combining quantities based on the correlation data for a plurality of reference segments distributed throughout the signal burst.
According to yet another aspect of the present invention, a method is useful for synchronization in a digital communication system transmitting a signal burst modulated by a modulation scheme that induces memory therein. The method comprises the steps of comparing the signal burst with a plurality of possible reference waveforms to generate comparison data not reflecting the memory induced by the modulation scheme and combining quantities based on the comparison data non-coherently to determine a timing offset for the signal burst.
According to yet another aspect of the present invention, a method is useful for estimating a frequency of a signal burst transmitted in a digital communication system wherein the signal burst comprises a plurality of reference segments. The method comprises the steps of calculating, for each reference segment, correlation data representative of an amount of correlation between the signal burst and one of a plurality of reference signals offset by a plurality of time offsets. The method further includes the step of generating a frequency domain representation of the correlation data associated with a time offset of the plurality of time offsets yielding a maximum amount of correlation over the plurality of reference segments. The frequency at which the frequency domain representation of the correlation data is a maximum is determined.